1. Field of the Invention
The present invention relates to a magnesium-based composite material that may be used to manufacture implanted medical devices, along with a mixer and a process used to manufacture the composite material.
2. Description of the Related Art
Numerous types of implanted devices, such as plates, screws, pins, stents, rods, anchors, and staples are used in orthopedic, spinal and vascular surgery, e.g., for bone fracture fixation, ankle stabilization, replacement of intervertebral discs, and cardiovascular inflation. The need of such implanted devices is huge and growing with an aging population. For example, for anterior cruciate ligament reconstruction only, more than 90,000 surgeries performed annually worldwide.
Currently, most implanted surgical devices are made primarily of titanium and stainless steel. While these materials have sufficient strength and rigidity to allow the healing process to begin, they are essentially neutral in vivo and are not biodegradable. These permanent fixtures result in adverse effects, such as tissue growth restriction, accumulation of metals in tissues, implant palpability, potential for cross contamination, among other negative impacts. Often, a secondary operation is needed to remove the devices, bringing more pain to the patient. This has led to increasing interest in the development of biodegradable polymer implants, made mainly of poly(lactic acid), poly(glycolic acid) and their copolymers. Biodegradable polymer implants have many advantages, e.g., no long-term implant palpability and temperature sensitivity, predictable degradation, and no stress shielding, leading to better tissue healing, reduced patient trauma, elimination of second surgery for implant removal, and no imaging interference. However, the relatively poor mechanical properties of polymer implants generate frequent surgical failures during installation and/or subsequent use. While efforts have been made to improve the mechanical properties of biopolymers, the effects are moderate since the basic stiffness and strength of polymers are orders of magnitude lower than those of metals. For this reason, polymer implants are not used widely or have limited use in less critical, secondary fixation applications.
Magnesium (Mg) has great potential as a strong and biodegradable material for implant fixation applications because the metal biodegrades and has mechanical properties matching the natural skeletal structure of the human body. The primary problem hampering the actual use of Mg in implanted devices is its extremely rapid degradation rate, losing mechanical integrity within a couple of weeks, rather than the months required for a tissue to fully heal. Furthermore, hydrogen gas is produced as a byproduct of the degradation process, which in small quantities is harmless, but can overwhelm host tissue if produced too quickly.
The primary factor behind rapid Mg-degradation rates is that the resulting oxides are porous, non-stoichiometric, and conduct electricity. By comparison, metals like aluminum, titanium, chromium, and tantalum form oxides that are non-porous, stoichiometric and do not conduct electricity, thus they are corrosion-resistant. Although alloying can be used to reduce degradation rates of Mg, known formulations of these Mg alloys include aluminum or other toxic elements, and therefore are not suitable for medical applications. Alternatively, organic and inorganic materials can be used to coat implants and delay degradation. For example, calcium phosphates (CaP) including hydroxyapatite (HA), tri-calcium phosphate, and calcium hydrogen phosphate (CaHP) have been found to be effective coating materials for Mg. However, coating alone does not lead to desired biodegradable devices; because once the coating layer is consumed, rapid degradation starts with the generation of large amounts of hydrogen gas. Therefore, magnesium-based materials with tunable degradation rates are needed.